The LD20 will be 2 and a 1/2 times brighter than the 3 watt Energizer I'm thinking of. I've yet to see a 2aa energizer pump out more than 60 to 80 lumens (emitter lumens)
The LD10 and LD20 are solid and well built lights, with a simple ui. I dont know that you need to buy both as there is only a 40 lumen difference between the two. Ld10 is 140 lumens on an energizer lithium, and the LD20 will be 180 lumens.
I'd get the Fenix LD10 and a Thrunite Neutron 2A. The user interfaces are similar. You'll have the 40% increase needed to see the improvement and you'll have more overall brightness options.
Fenix makes great lights, but buying two near identical lights isn't as fun as two from different companies in my opinion